Triple-quadrupole mass spectrometry (TQ-MS) has been frequently used in combination with high-performance liquid chromatography (HPLC) interfaced with the electrospray ionization (ESI) method (see Siuzdak, G. Proc. Natl. Acad. Sci. USA 1994, 91, 11290-11297; Yu, X., Cui, D., Davis, M. R. J. Am. Soc. Mass Spectrom. 1999, 10, 175-183; Gygi, S. P., Aebersold, R. Chem. Biol. 2000, 4, 489-494; Villas-Boas, S. G., Mas, S., Akesson, M., Smedsgaard, J., Nielsen, J. Mass Spectrom. Rev. 2005, 24, 613-646; Korfmacher, W. A. Drug Discovery Today 2005, 10, 1357-1367). Such a liquid chromatography-mass spectrometry (LC-MS) system can be used to acquire structural information, as it facilitates the analysis of the fragmentation of an ion under collision-induced dissociation (CID) conditions. Furthermore, from a retention time of a given species in the chromatography column, it is possible to determine affinity of the species for a stationary phase used in the HPLC and to thus acquire information on the structure. However, this technique is disadvantageous that said technique provides only MS/MS information. Additionally, Quadrupole ion trap mass spectrometry (QIT-MS) allows for multistage MS/MS analysis which provides in-depth structural information (see Gygi, S. P., Aebersold, R. Chem. Biol. 2000, 4, 489-494; Villas-Boas, S. G., Mas, S., Akesson, M., Smedsgaard, J., Nielsen, J. Mass Spectrom. Rev. 2005, 24, 613-646; March, R. E. Mass Spectrom. Rev. 2009, 28, 961-989; Jonscher1, K. R., Yates III, J. R. Anal. Biochem. 1997, 244, 1-15; March R. E. Int. J. Mass Spectrom. 2000, 200, 285-312; March R. E. Rapid Commun. Mass Spectrom. 1998, 12, 1543-1554). However, it is practically difficult to perform multistage MS/MS experiments during the period of elution of a compound in the HPLC analysis. Accordingly, in order to gain new insights into structural information within time required for the HPLC analysis, it is important to overcome a disadvantage of TQ-MS, which does not allow for sequential mass spectral analysis (MSn).
In the past, energy-resolved mass spectrometry (ERMS) using QIT-MS has been investigated in order to develop a method elucidating structures of complex glycans and configurations of glycosidic linkages. In most cases, ERMS spectra acquired for sodiated ions of various oligosaccharides are simple, with a series of peaks corresponding to a precursor ion and a plurality of product ions. These peaks are analyzed and approximated by using Boltzmann sigmoidal equations (see Kurimoto, A., Daikoku, S., Mutsuga, S., Kanie, O. Anal. Chem. 2006, 78, 3461-3466; Daikoku, S., Ako, T., Kato, R., Ohtsuka, I., Kanie, O. J. Am. Soc. Mass Spectrom. 2007, 18, 1873-1879; Shioiri, Y., Suzuki, K., Kanie, O. J. Mass Spectrom. 2008, 43, 1132-1139; Shioiri, Y., Kurimoto, A., Ako, T., Daikoku, S., Ohtake, A., Ishida, H., Kiso, M., Suzuki, K., Kanie, O. Adnal. Chem. 2009, 81, 139-145). More complex ERMS spectra may be acquired in rare cases. In the course of elucidating gas-phase reactions of glycans, it has been revealed that a complex spectrum was acquired therein and that the MSn spectrum contained some information regarding fragmentation reactions of a product ion (information of MSn+1) which was not usually acquired in a CID process of QIT-MS. Such information, however, is considered less important for a QIT-MS analysis because a QIT-MS apparatus can be used to perform MSn experiments. In contrast, the aforementioned MSn+1 information will be very important for a TQ-MS analysis, as a TQ-MS apparatus cannot be used for MSn experiments. Additionally, such MSn+1 information would also be useful when the TQ-MS method is used in combination with HPLC for structure elucidation.